1. Field of the Invention
The present invention relates to a coaxial cable having a laterally wound shielding layer for use in, for example, an ultrasonic diagnostic device.
2. Description of the Prior Art
In an ordinary coaxial cable, a woven metallic member is used as a shielding layer to enhance shielding characteristics for the purpose of increasing surface density of the shielding layer. Dual woven metallic layers for this purpose are generally known. In a conventional structure of this type, however, the outer diameter of the cable becomes large, and sufficient flexibility of the cable has not been obtainable.
To remedy these problems, a plurality of copper wires have been spirally wound to provide a laterally wound shielding layer as the shielding layer in order to reduce the outer diameter of the cable and yet provide a given flexibility. Such a coaxial cable is available if it is used for low frequency bandwidths around 1 MHz, for example. However, the laterally wound shield does not provide a sufficient shielding characteristic due to the continuous slide of the copper wires, and the resultant coaxial cable is insufficiently shielded when used with an ultrasonic diagnostic device which requires a bandwidth of 10 MH or more. Therefore, a coaxial cable having a minimized outer diameter, yet providing a sufficient shielding characteristic against high frequency bandwidths, has not heretofore between realized.
In order to overcome the above-mentioned drawbacks, it is conceivable to design a coaxial cable in which the laterally wound shielding layer is wound with an aluminum foil tape or with a composite tape in which the aluminum foil is adhered on the plastic tape. However, the latter type of coaxial cable is not sufficiently flexible; therefore, the aluminum foil may be broken and the shielding characteristic may be degraded over time. Such problems are compounded if the cable is used in a diagnostic device which requires a severe bending condition of the cable.